The invention relates to the field of gliding sports and more precisely to that of alpine skiing. It relates to a board geometry for a small ski which is nevertheless particularly manoeuvrable whilst retaining a behaviour which is substantially equivalent to that of a ski of conventional size.
Conventionally, the optimum length of a ski is determined as a function of the height of the user, and of the latter""s weight and technical ability.
Therefore, current opinion holds that a ski must have a length which is substantially between approximately 10 and 20 centimeters longer than the height of the user.
In practice, the longer a ski, the more it tends to keep to its course and to allow precise skiing. Conversely, the shorter a ski, the more frequently floating phenomena are observed, particularly when skiing at high speed.
Nevertheless, the greater length of the skis makes them more difficult to manoeuvre and requires more effort on the part of the skier, particularly when skiing turns.
Thus, it is known that the issue of the size of a ski must make it possible to obtain a compromise between manoeuvrability and skiing precision.
Numerous developments in the geometry and determination of the lengths of skis have already been proposed, but these have not made it possible to achieve optimum solutions.
It has thus already been proposed, within a range of skis called xe2x80x9ccompact skisxe2x80x9d to greatly reduce the size of a ski, by approximately 20 centimeters with respect to the conventional size, and to make the tip wider. Such skis were intended for use by intermediate skiers, to allow versatile skiing. These skis were fairly easy to ski on but gave a relatively poor performance.
Further developments in the field of skiing were also proposed in document FR 2 559 673, consisting particularly in making very deep sidecuts to allow turns to be carved. This practice requires very good physical condition. Such skis are thus relatively difficult to ski on, unstable in a straight line, when the skis are flat, and are not versatile.
Another parameter involved in the design of a ski is its stiffness in terms of flexion. This stiffness makes it possible to distribute the skier""s weight over the snow. It is defined more or less empirically by ski manufacturers in such a manner that, with the ski placed flat, the maximum load is located under the skier""s feet, this load diminishing towards the ends of the ski.
These various parameters are involved in the production of a ski, usually with paradoxical consequences. Therefore:
a reduction in the length may give rise to a reduction in the weight and thus the stability of the ski, but also a reduction in the inertia, enhancing its manoeuvrability;
a small side line radius leads to an increase in the mass at the ends of the ski, therefore to an increase in its inertia, but also to an interference with the stability of the ski when it is flat on the snow.
An attempt was made, particularly in document U.S. Pat. No. 5,603,522, to determine the load-bearing surface of the ski with respect to its polar moment of inertia in order to improve its manoeuvrability by reducing the forces necessary for turn initiation. The load-bearing surface is a parameter which is involved only when the ski is flat. In point of fact, during a turn, a ski bears on its edge line, so this document provides no useful teaching for improving the behaviour during a turn.
One of the objectives of the invention is to provide a ski which is shorter than the skis which are conventionally used for a given user height, which allows satisfactory precision in skiing via the effective transmission of the forces exerted by the skier over the entire length of the side line.
Side line or edge line is understood to mean the curve defined by the sharp portion of the edge from one end to the other of the ski. It is generally measured by determining the distance variation of the sharp portion of the edge with respect to the longitudinal mid plane of the ski.
The invention thus relates to an alpine ski which is broken down over its length into a tip area, a binding area, and a heel area, and in which the side line is such that the binding area has a minimum width level, the tip area has a front maximum width level, and the heel area has a rear maximum width level.
An alpine ski according to the invention is characterized by the combination of a plurality of dimensional mechanical parameters which thus make it possible to achieve the same behaviour as conventional skis which are approximately 20 centimeters longer, but thus at the same time to enhance the manoeuvrability of the ski.
Thus, according to the invention, the radius of the side line, calculated on the basis of three points on the side line which are located respectively at the rear maximum width level, and the front maximum width level and centrally between these two levels, is between 7 and 21 meters.
Moreover, the pressure distribution on the edge is one of the predominant parameters in the satisfactory operation of a ski, i.e. in its ability to initiate a turn and to keep to its course without skidding or chattering.
The ski according to the invention therefore has an edge pressure distribution along the side line such that, when the ski is placed on a flat surface, so that its underside forms an angle of 45xc2x0 with the said flat surface, and when the ski receives, at the location of the center of the boot, a force of 400 Newtons perpendicularly to its gliding surface, the pressures measured along the side line differ by less than 10% from the average value of the three pressures measured respectively at the rear maximum width level, at the minimum width level of the binding area, and at the front maximum width level.
In other words, by virtue of the ski according to the invention, the forces exerted by the skier, particularly when executing a turn, are very regularly and quasi uniformly distributed along the edge line, which ensures optimum skiing of the turn.
In point of fact, the skis of the prior art have a pressure distribution on the edge line which is such that the majority of the forces are transmitted to the level of the binding area, and more precisely in line with the boot, whilst the front and rear areas of the board transmit only a very small portion of the forces.
This stiffness adjustment is obtained correctly either by adding reinforcements in the appropriate areas or by adjusting the thickness of the ski so as to vary the distance of the reinforcements with respect to the neutral fiber.
Advantageously, in practice, the distribution of pressures along the edge line is such that the pressure value measured at the location of the maximum width of the heel is slightly greater than the pressure value measured at the level of the maximum width of the tip.
In other words, the forces exerted by the skier during a turn are distributed quasi uniformly between the front and the rear of the ski since, during a turn, the force exerted by the skier is positioned in front of the center of the boot, which offsets the characteristic pressure distribution determined with a static load located exactly at the level of the center of the boot.
It was observed that the behaviour of the ski was very favorable when the pressure value measured at the level of the maximum width of the heel is greater by approximately 10% than the pressure value measured at the maximum width level of the tip.
Moreover, the skis according to the invention are shorter than conventional skis. However, they are not equivalent to shorter conventional skis, i.e. the skis normally used either by short people or by children, or by persons of low weight or of low muscle mass.
In fact, the ski according to the invention must be stiffer in terms of flexion in order to withstand considerable loads and more powerful impulses.
Thus, it was determined that, when considering the load-bearing length of the ski defined between the front contact lines and the rear contact lines, the stiffness of the ski according to the invention is such that, when the ski is placed flat between two supports, and when a force of 400 Newtons is exerted perpendicularly to the upper face of the ski midway between the two supports, the point located midway between the two supports is displaced downwards with respect to the situation in which the load is absent
by a distance of between 60 and 70 millimeters when the supports are located respectively at the rear contact line and at {fraction (5/18)}ths of the load-bearing length measuring from the rear contact line;
by a distance of between 50 and 60 millimeters when the supports are located respectively at {fraction (5/18)}ths of the load-bearing length measuring from the rear contact line and at {fraction (13/18)}ths of the load-bearing length measuring from the rear contact line;
by a distance of between 65 and 75 millimeters when the supports are located respectively at the front contact line and at {fraction (5/18)}ths of the load-bearing length measuring from the front contact line.
Such stiffness thus helps to give the shorter ski a behaviour pattern which is equivalent to that of a longer ski.
In practice, the skis according to the invention have a total length, measured between the front and rear ends of the ski, of between 1 300 and 1 740 millimeters.
This is a range which corresponds to a range of conventional skis which are approximately 20 centimeters longer.
Advantageously, in practice, the ski according to the invention has a side line such that:
its width measured at the front maximum width level is between 102 and 108 millimeters;
its length measured at the minimum width level is between 64 and 70 millimeters;
its width measured at the rear maximum width level is between 92 and 100 millimeters.